Flexible supercapacitor sets new high for volumetric energy density

16 May 2014

Researchers at Nanyang Technological University (NTU) in Singapore, Tsinghua University in China, and Case Western Reserve University have developed a flexible micro-scale supercapacitor with what they believe is the highest reported volumetric energy density for carbon-based microscale supercapacitors to date: 6.3 µWh per cubic millimeter.

The hierarchically structured carbon microfiber made of an interconnected network of aligned single-walled carbon nanotubes with interposed nitrogen-doped reduced graphene oxide sheets, and stores energy comparable to some thin-film lithium batteries—an area where batteries have traditionally held a large advantage, while also maintaining high power density.

The nanomaterials form mesoporous structures of large specific surface area (396 m2 g−1) and high electrical conductivity (102 S cm−1). … The resultant fibres show a specific volumetric capacity as high as 305 F cm−3 in sulphuric acid (measured at 73.5 mA cm−3 in a three-electrode cell) or 300 F cm−3 in polyvinyl alcohol (PVA)/H3PO4 electrolyte (measured at 26.7 mA cm−3 in a two-electrode cell).

A full micro-supercapacitor with PVA/H3PO4 gel electrolyte, free from binder, current collector and separator, has a volumetric energy density of ~6.3 mWh cm−3 (a value comparable to that of 4 V–500 µAh thin-film lithium batteries) while maintaining a power density more than two orders of magnitude higher than that of batteries, as well as a long cycle life. To demonstrate that our fibre-based, all-solid-state micro-supercapacitors can be easily integrated into miniaturized flexible devices, we use them to power an ultraviolet photodetector and a light-emitting diode.

The researchers have also developed a way continuously to produce the flexible fiber, enabling them to scale up production for a variety of uses. To date, they’ve made 50-meter long fibers, and see no limits on length.

They envision the fiber supercapacitor could be woven into clothing to power medical devices for people at home, or communications devices for soldiers in the field. Or, they say, the fiber could be a space-saving power source and serve as “energy-carrying wires” in medical implants.

The scientists report their research in Nature Nanotechnology.

In testing, they found that three pairs of fibers arranged in series tripled the voltage while keeping the charging/discharging time the same. Three pairs of fibers in parallel tripled the output current and tripled the charging/discharging time, compared to a single fiber operated at the same current density.

When they integrate multiple pairs of fibers between two electrodes, the ability to store electricity, called capacitance, increased linearly according to the number of fibers used.

We have tested the fiber device for 10,000 charge/discharge cycles, and the device retains about 93 percent of its original performance, while conventional rechargeable batteries have a lifetime of less than 1000 cycles.

—Dingshan Yu, lead author

The team also tested the device for flexible energy storage. The device was subjected to constant mechanical stress and its performance was evaluated. The fiber supercapacitor continues to work without performance loss, even after bending hundreds of times, Yu said.

The researchers plan to scale up the technology for low-cost, mass production of the fibers aimed at commercializing high-performance micro-supercapacitors. The team is also interested in testing these fibers for multifunctional applications, including batteries, solar cells, biofuel cells, and sensors for flexible and wearable optoelectronic systems.

The Ministry of Education, Singapore and Asian Office of Aerospace Research and Development of the US Air Force and the US Air Force Office of Scientific Research funded the research.